Lead/acid storage batteries are the principal means for storing electrical energy for use in vehicles and industrial applications. Although such batteries typically have service lives measured in years, they eventually become unable to take or hold a charge and must be replaced with new batteries.
The lead content of the grids, plates, and mud of spent lead/acid storage batteries can be readily recovered by processing these materials through the furnaces of a secondary lead smelter. The lead content of any materials remaining on battery casings made of non-porous plastic (polypropylene), can also be recovered relatively easily by simply washing the plastic battery casing material with an aqueous detergent followed by a water rinse. The plastic battery casing material can then be recycled.
Today, most battery casings for lead/acid storage batteries are made from plastic. However, a significant number of battery casings were and are still being produced from ebonite, or "hard rubber." Although the composition of ebonite has varied slightly among manufacturers, a generic composition of typical ebonite is shown in Table I.
TABLE I ______________________________________ Typical Components of Ebonite Battery Casing Materials. (Components are listed in order of decreasing weight percentage in an average collection of these materials.) ______________________________________ gilsonite (a natural carbonaceous material) or powdered coal thermoplastic resin binder bituminous asphalt non-asbestos fibrous materials and filler fibers lampblack pigment mineral wax comminuted rubber ______________________________________
Ebonite is hard and black in color, and the addition of the small amount of comminuted rubber to ebonite led the industry to adopt the term "hard rubber" to identify this material.
Though strong and inexpensive, ebonite has been largely displaced by plastic in battery casing production because of the expense and difficulty associated with the disposal of lead-contaminated ebonite. Unlike plastic, ebonite is relatively porous and has small cracks and crevices on its surface, making it much more difficult to clean. During the useful life of a lead/acid storage battery that has an ebonite casing, lead-contaminated electrolyte works into the pores, cracks and crevices in the ebonite and deposits lead-bearing materials. When an ebonite battery casing is broken by a battery recycler, some of this lead-bearing material is readily mobilized, but much of it remains absorbed and adsorbed within the ebonite, significantly contaminating the material with combined lead compounds.
In the past, broken ebonite battery casings have been fed as fuel into specifically equipped furnaces at secondary lead smelters, with attendant lead recovery. However, because not all furnaces could readily accept ebonite battery casing material, very few recycling sites for lead-bearing ebonite actually existed. Before the implementation of federal hazardous waste landfill rules, secondary lead smelter managers who where recycling spent lead/acid storage batteries found it simpler and cheaper to discard ebonite battery casing material on their own sites rather than to equip their furnaces to recycle it. There are, therefore, sizable accumulations of discarded, lead-contaminated ebonite battery casing material at numerous former battery recycling sites.
According to the U.S. Environmental Protection Agency (EPA), there are at least 25 major Superfund sites in the United States at which the principal hazardous material preventing effective site cleanup is lead contaminated ebonite battery casing material. These Superfund sites are large, and contain millions of square feet of lead contaminated ebonite battery casing material and lead contaminated soil and other materials. An exemplary composite analysis of wastes from such sites is shown in Table II.
TABLE II ______________________________________ Components Identified in Ebonite Battery Casing Pile Materials Gathered at Two Abandoned Secondary Lead Smelter Sites. Component Percentage ______________________________________ ebonite 87.4 lead oxides 4.8 plastic 2.0 elemental lead (grids) 1.9 fine material, not identified 1.6 soil, rocks 1.1 paper 0.8 wood 0.4 separators 0.4 ______________________________________
Careful examination of the lead-containing materials in the ebonite battery casing piles, chemical analysis, and literature searches indicated the presence of a wide variety of lead compounds at abandoned secondary lead smelter sites, predominately the more easily handled common oxide and sulfate. Very little lead dioxide or lead tetroxide was found to exist in any of the many samples tested from waste sites, making cleanup of such sites easier.
Analysis of the lead contamination within spent ebonite battery casings, whether taken from waste site materials or from freshly broken batteries, is accomplished in one of two ways. The total lead content of ebonite battery casing material is determined by digesting the material thoroughly in a strong oxidizing acid, such as concentrated nitric acid. This oxidizes much of the case material and mobilizes essentially all of the lead contamination, which can then be quantified by solution spectroscopic methods (typically atomic absorption spectrometry). For determining the environmental hazard posed by this waste material, the EPA has developed two different extractable lead methods. The older EP Tox (Extraction Procedure for Toxicity Characteristic) test used manual additions of acetic acid to the waste, while the newer TCLP test (Toxicity Characteristic Leaching Procedure) uses treatment with an acetic acid buffer. Although the two tests give slightly different results, they employ the same basic chemistry and the criterion for judging the material hazardous by each is identical: if the final leachate has more than 5 mg/L lead, the waste is hazardous.
Typical values found at Superfund sites where broken ebonite battery casing material has been tested by these methods are, a total lead content of 5000 mg/kg and an EP Tox (TCLP leachate) value of 50 mg/L. EPA regulations allow battery breakers to sell or utilize freshly broken ebonite battery casing material as a nonhazardous material if the EP Tox (TCLP leachate) test results can be reduced to below 5 mg/L. At Superfund sites, Applicants have been advised that the EPA considers the cleanup complete if the total lead content of the site's ebonite battery casing material can be reduced to less than 500 mg/kg and if the EP Tox (TCLP leachate) value can be brought below 5 mg/L.
Today, there are a number of secondary lead smelters with battery recycling operations that still discard broken ebonite battery casing material, even though the material must now be shipped to a licensed hazardous waste landfill with associated high transportation and disposal costs. The high costs of licensed disposal of ebonite battery casing material, and the high costs associated with a successful Superfund site cleanup, have created major economic incentives for a process for recovering lead from broken ebonite battery casing material, but to date there has been no demonstrated method for successfully performing these tasks.
An electrowinning technology works for removing lead contaminates only in a narrowly defined concentration range. Outside of the narrow concentration ranges most useful for this technology, electrowinning performs poorly. Several battery recyclers have attempted to utilize electrowinning technology to recycle lead contaminated broken ebonite battery casing material, but have now abandoned this technology. In addition to its poor performance, electrowinning uses a dangerous chemical and consumes vast amounts of electrical energy.
A thermotreatment/leaching method for recycling lead contaminated broken ebonite battery casing material is being developed by the United States Bureau of Mines. In this method, the lead contaminated ebonite battery casing material is burned, which converts the lead oxides and other lead contaminate compounds into carbonates. The ash containing the carbonates is then leached. The levels of lead removal achieved by this method have been poor by comparison to the process of the present invention, and the burning of ebonite battery casing material introduces the possibility of air pollution. More specifically, to date, this method has failed to meet the lead contamination cleanup standards set by the EPA discussed above.
An environmentally sound method is needed to clean up the present problems that have been created by discarded lead-contaminated ebonite battery casing material, to prevent new problems from arising, and to produce a recycled ebonite product that can re-enter the market as a safe, usable material. Clean ebonite casing material is potentially valuable as a material resource or as a fuel. Standard testing procedures indicate a heat content of 13,000 BTU/lb, which is comparable with Indiana coal. It can be pelletized, powdered, or otherwise easily prepared for combustion in any furnace that can utilize coal as a fuel. Moreover, the cleaned material can also be used as a filler in roads, other asphalt applications, and a variety of other products. Up to 3% of the recycled material can be incorporated into new battery casings.